Optical fiber has become a widely accepted form of transmission media. The use of optical communications involving the use of optical fibers has grown at an unprecedented pace. A continuous length of the fiber is drawn from an optical preform which may be made by any one of several known processes. Afterwards, or as part of a tandem process, the drawn fiber is coated, cured, measured and taken up, desirably in an automatic takeup apparatus, on a spool. Methods and apparatus for taking up optical fiber are disclosed and claimed in U.S. Pat. No. 4,798,346 which issued on Jan. 17, 1989 in the names of D. L. Myers and J. W. Wright. Typically, an optical fiber has a diameter on the order of 125 microns, and is covered with a coating material which increases the outer diameter of the coated fiber to about 250 microns, for example.
The spool on which the optical fiber is taken up has other uses. It is used to store the optical fiber, to pay out and to take up the fiber for other operations such as ribboning, cabling, and rewinding and is used to ship optical fiber which is wound thereon to other companies which further process the fiber. Also, it may be used in weapons and communications systems in which it may be attached to a control station.
Weapons and communications systems which use an optical fiber for two-way data communication between two or more moving bodies or between a moving body and a guidance station have been developed or ar under development. Such uses include communication lines between aircraft, between an aircraft and a ship, and between a projectile, such as a missile, and a control station at a launch site, for example. Advantageously, the use of optical fiber for these kinds of communication precludes electomagnetic interference and undesired interception. There are, however, certain disadvantages, not present in other other forms of communication in using optical fiber. Optical fiber is less robust than metallic conductors, rendering it subject to breakage. Aside from breakage, optical fiber communication performance may be degraded by microbends in the fiber which are generated by bending or by other stresses to which the fiber is subjected. Such damage to an optical fiber not only reduces the long-term durability of the fiber, but also causes losses in the strength and in the content of the optical signal.
A typical optical fiber application in a weapons systems involves the packaging of a continuous length of optical fiber on a carrier bobbin which is positioned inside a vehicle. Such a vehicle commonly is referred to as a tethered vehicle. One end of the fiber is attached to operational devices in the vehicle, whereas the other end of the fiber is connected to a control or communications station at the launch site. During and after launch, two-way communication with the vehicle is conducted.
In order to use such an arrangement, there must be provided a reliable and compact package of the optical fiber which may be disposed within the vehicle and which will permit reliable deployment of the optical fiber during the flight of the vehicle. The use of metallic conductors for guidance or control of launched vehicles is known. See, for example, U.S. Pat. Nos. 3,114,456, 3,156,185 and 3,319,781. As mentioned hereinabove, the characteristics of optical fiber present difficulties not involved in the use of metallic conductors for communication. Specialized treatment is required to facilitate the unwinding of the optical fiber its carrier bobbin at a relatively high rate of speed.
One problem is that the introduction of optical fiber for use in more hostile environments, such as in underwater cable or in military applications, has required that more stringent requirements be imposed on the physical properties of the fiber. Additionally, extremely long lengths of fiber may be required and may be obtained by splicing a plurality of lengths which are obtained using current manufacturing techniques. For these and other applications, splicing, in which the coating material is removed from end portions of two fibers which are then fused together end to end, provides a suitable means for joining the ends of two glass fibers with an acceptable loss.
Bared spliced fiber end portions must be recoated, maintaining stringent requirements on dimensional and strength parameters associated with the coated fiber. Typically, the recoating material contacts the adjacent originally coated portions of the spliced fibers along substantially radial planes exposed when the original coating material was removed from the end portions and along overlapping portions of the outer surface of the original coating material adjacent to the radial planes. The coating material is then cured to yield a recoated splice section with a transverse cross section which is larger than that of the optical fiber having the original coating material thereon.
In a typical tethered vehicle, an optical fiber which is wound on a payoff device and connected to a guidance system is payed off as the vehicle is moved. For tethered vehicles, the winding of the optical fiber on the payoff device must be accomplished in a precision manner. Otherwise, payoff could be disrupted. It has been found that if the cross section of the recoated spliced portion transverse of the longitudinal axis of the optical fiber is not the same as that of the optical fiber as originally coated, the winding pattern on the payoff device in all likelihood is not uniform. This will cause problems in fiber payoff following the launch of the tethered vehicle. This problem has been solved. A recoated splice having the same transverse cross section as that of the unspliced fiber has been attained by the use of methods and apparatus disclosed in application Ser. No. 133,579 which was filed on Dec. 16, 1987 in the names of R. J. Darsey, et al.
Another problem in the optical fiber guidance of tethered vehicles relates to the successful unwinding of the fiber from a carrier bobbin as the bobbin is propelled along with the vehicle. The leading end of the optical fiber is connected to a guidance system for controlling the path of travel of the vehicle. It becomes important for the optical fiber to be payed off from the bobbin without the occurrence of snags, otherwise the fiber may break and the control system rendered inoperable. Contributing to the successful payout of the optical fiber is a precision wound package. Further, not only must the convolutions be wound with precision, they also must remain in place as wound during handling and when deployed. In other words, the optical fiber package must be a highly stable one. On the other hand, payout must occur easily without the necessity of high pulling forces to remove each convolution of fiber from the carrier bobbin.
In optical fiber packages for use in tethered vehicles, as many as thirty layers of optical fiber are wound on a base layer of wire. An adhesive material between the optical fiber turns functions to hold the package together, forming a stable structure which is resistant to environmental extremes, shock and vibration. Desirably, the adhesive material which is used to hold together the convolutions must have a minimal impact on the optical performance of the wound optical fiber, and yet it must allow the optical fiber to be payed out with a controlled force at the peel-off point as the outermost turn is unwound at high speed. These requirements present somewhat conflicting requirements for the adhesive system.
During storage and transport of the carrier bobbin, mechanical stability is most important as the adhesive adds integrity to the wound package thereby maintaining the package in a ready condition for deployment. During deployment, both mechanical and optical effects are significant. The adhesive system must provide tackiness which is sufficiently low to permit a helical pattern of payout at speeds which may be relatively low to speeds which may be in the supersonic range. Excessive tackiness threatens fiber integrity by forming an extreme bend at the peeloff point. On the other hand, not enough tack may result in failure through dynamic instability on the bobbin surface. With respect to optical performance, optical attenuation at the peel offpoint of each successive convolution may occur through localized macrobending, degrading the integrity of data and video transmission. Typical peel-off point attenuation of each successive convolution may contribute 3 or more dB to the overall loss.
Also, it has been found that microbending in the layers of undeployed fiber in the bobbin during deployment can affect adversely optical performance. It has been found that the adhesive material can contribute significantly to attenuation increases, especially at lower temperatures.
Current techniques for providing a sought-after stable package include the coating of each layer of fiber convolutions as they are or after they have been wound on the bobbin. In the prior art, at least one system includes a spraying apparatus. The apparatus is used to apply a wet adhesive to the optical fiber convolutions as they are wound on a bobbin. Depending on how the adhesive of this system is applied, the application may not be duplicatable from one bobbin to another. Needless to say, this is an expensive procedure necessitating perhaps the interruption of the winding operation after each layer to allow the coating to cure or solvent to evaporate.
Another technique involves the winding of the convolutions on a carrier bobbin followed by the impregnation of the wound fiber with a curable material having a low tear strength. When the impregnating material cures, it gels and holds the convolutions together in a precision wound package. During payout, the cured material is torn apart, releasing each convolution from its neighbor. The force developed during payout is related to the tear strength of the cured material.
What still is needed are a system which includes more reliable, precision wound bobbins of optical fiber in which the convolutions of fiber are held together by an adhesive material. The adhesive material should be such that it stabilizes the package yet permits payout at relatively high speeds. Further, the process should be easily repeatable from one bobbin to another.